The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of a rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos Θ, where Θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. A pinned layer in an AP pinned spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second non-magnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Sensors can also be categorized as current in plane (CIP) sensors or as current perpendicular to plane (CPP) sensors. In a CIP sensor, current flows from one side of the sensor to the other side parallel to the planes of the materials making up the sensor. Conversely, in a CPP sensor the sense current flows from the top of the sensor to the bottom of the sensor perpendicular to the plane of the layers of material making up the sensor. In a CPP sensor design, the magnetic shields usually double as electrical leads for supplying a sense current to the sensor. Therefore, in CPP sensor design, the shields/leads contact the top and bottom of the sensor.
The ever increasing demand for data storage density and data rate have increasingly pushed the limits of data storage designs. Recently in efforts to overcome such limits, engineers and scientists have focused on the use of perpendicular recording. In a perpendicular recording system a write pole emits a highly concentrated magnetic field that is directed perpendicular to the surface of the medium (eg. the disk). This field in turn magnetizes a localized portion of the disk in a direction perpendicular to the surface of the disk, thereby creating a bit of data. The resulting flux travels through the disk to a return path having a much larger area than the area in which the bit was recorded. The increased interest in perpendicular recording has lead to an increased interest in current perpendicular to plane (CPP) sensors, which are particularly suited to use in perpendicular recording.
The development of perpendicular recording systems have presented several challenges. For example, as discussed above, when using a GMR or AMR sensor, the sensor must be disposed between a pair of magnetic shields in order to avoid reading stray fields and to define the bit length (gap height). However, in a perpendicular recording system, due to the bi-layer nature of the recording medium the use of shields can actually erase data from the disk. Because the disk in a perpendicular recording system has a magnetically soft under-layer, the shields tend to act sort of as magnetic antennas that concentrate stray longitudinal and transverse magnetic fields that can inadvertently erase data from the disk.
Another challenge associated with perpendicular recording is the nature of the signal read from the disk. In a longitudinal system, the signal read resembles a bell curve, and the algorithms currently in use are adapted to read such bell curves. In a perpendicular recording system however, the signal is bi-polar in that it resembles a sine wave that passes from positive to negative for a single bit of data. This presents challenges for read channel designers in that new algorithms must be developed to read the new signal curve.
With the ever increasing need for increased data density and data rate, a strong need exists for decreasing bit lengths in order to fit more bits of data onto a given length of data track. As those skilled in the art will recognize, the bit length when using a GMR or AMR sensor is limited to the distance between the shields. One way to greatly decrease the bit length is to use a differential sensor. A differential sensor essentially comprises a pair of GMR sensors, the free layer of each sensor being separated by a spacer layer. The spacer layer can be constructed of a non-magnetic material such as Cu and need only be thick enough to prevent magnetic coupling of the two free layers. The pinned layers are then located opposite one another at opposite sides of the dual GMR structure. The pinned layers each have a reference layer, which is the portion of the pinned layer closest to its respective free layer and is the portion of the pinned layer that determines the GMR effect. In such a differential structure, the reference layers of the GMRs are out of phase with one another. That is to say they have magnetic moments that are pinned 180 degrees with respect to on another. In this way, when the free layers of each GMR are detecting the same magnetic field (eg. magnetic field oriented in the same direction) the signals from each GMR cancel out. However, when one free layer is detecting a field in one direction, and the other free layer detects a field in the opposite direction, the signals from each GMR are additive. In this way the differential GMR can read a magnetic transition on a magnetic medium. When the differential GMR passes over such a transition, it will register a resistance change when each free layer is on an opposite side of the transition.
The effective read gap (ie. bit length) when using a differential GMR sensor is the distance between the first and second free layers, a distance which can be exceedingly small. In fact the read gap of a differential sensor can be a small fraction of that which is possible using a standard GMR sensor. Another advantage of using a differential GMR sensor is that no shields are needed. This eliminates the above discussed problem of disk erasure. Such a differential sensor also has the advantage that it reads a magnetic transition in a perpendicular recording system as a bell curve rather than a bipolar sine wage, thereby avoiding the need to create new channel algorithms as discussed above.
One problem that exists with prior art differential sensors is that stray longitudinal fields from adjacent tracks can be read by the sensor, thereby generating unacceptable noise in the signal. This problem becomes more acute as track density increases.
Therefore, there remains a need for a practical differential GMR sensor design that can reduce or eliminate noise produced by stray longitudinal fields such as from adjacent tracks. Such a design would preferably provide enhanced GMR signal, since such performance enhancements are needed to meet ever increasing data rate and data density requirements. Such a design would also preferably be usable as a CPP sensor useful in perpendicular recording systems and could eliminate the need for magnetic shields.